The disclosure relates to an air seal for a gas turbine engine.
In compressor and turbine sections of a gas turbine engine, air seals are used to seal the interface between rotating structure, such as a hub or a blade, and fixed structure, such as a housing or a stator. For example, typically, circumferentially arranged blade seal segments are fastened to a housing, for example, to provide the seal.
Relatively rotating components of a gas turbine engine are not perfectly cylindrical or coaxial with one another during engine operation. As a result, the relatively rotating components may occasionally rub against one another. To this end, an abradable material typically is adhered to the blade seal segments or full rings and/or the rotating component.
Abradable seals in the compressor section of gas turbine engines include characteristics such as, good abradability, spall resistance, and erosion resistance. Abradable seals are required to exhibit a smooth surface, low gas permeability, and environmental durability. The seal is a sacrificial element in order to minimize blade wear, so it is abradable. The seal must also minimize gas flow leakage through the seal, so a low gas permeability is desirable.
Abradable coatings for the seals are always a compromise between abradability and erosion resistance. In order to maintain blade tip clearances over time, the seal material needs to be tough and resistant to erosion. Conventional seal materials tend to be soft and weak in order to have good abradability. However, current understanding of the wear mechanisms involved with high pressure compressor (HPC) abradable materials, while rubbed with bare Nickel (Ni) alloy blades, is that wear takes place by adhesive wear mechanisms, plastic deformation and fracture on a scale far smaller than the size of coating constituent particles. This becomes apparent when one considers that the wear per blade passage is on the order of 1 E−6 inches.
The new understanding of the scale of wear particle removal provides insight that leads to an improved coating structure that minimizes volume fraction of the strong metal constituents while assembling them in a structure that maximizes their contribution to toughness and strength.